Abrasive tools come in many types, each generally designed for specific applications and no one type providing a universal abrading tool for all applications. The various types of abrading tools include, for example, coated abrasives, i.e., abrasive granules generally uniformly distributed over and adhered to the surface of a flexible backing; grinding wheels, i.e., abrasive material consolidated together in a mass in the form of a rotatable annulus; and low density abrasives, i.e., an open, lofty, three-dimensional fiber web impregnated with adhesive which does not alter the open character of the web and also adheres abrasive granules to the web.
While low density-type abrasive products have enjoyed considerable commercial success as metal, wood and plastic finishing tools, there are two areas in which this type of abrasive tool has had limited success because of its inability to achieve a high cut rate and/or to produce a level surface having a uniform scratch depth on the surface being abraded. Surfaces finished with low density abrasive typically exhibit a matte finish characterized by a non-uniform pattern of relatively deep and shallow scratches and not a polished, glossy finish. Thus, low density abrasive products have generally not been used in applications which require the production of surfaces which are buffable to a mirror-like finish similar to that which is produced by buffing and electroplating. Presently, the major portion of these tasks are accomplished by the use of coated abrasive belts or abrasive set up wheels, both of which have disadvantages.
A coated abrasive belt has a very high initial cut rate and produces a high surface roughness when new, but each of these properties drops off very rapidly with use. Coated abrasive belts also provide a very limited degree of conformability because of the manner in which they are supported in the abrading machine, limiting their use on complex surfaces. Soft back up wheels of various types are used with coated abrasives but the restricted stretchability of the coated abrasive backing limits the conformability of the belt.
Set up wheels are generally constructed from a stack of cotton discs which are compressed to a desired firmness and sewn together. The edge of the disc is then coated with an adhesive such as animal hide glue or a synthetic resin and, while the adhesive is still wet, the wheel is rolled through a bed of abrasive mineral and allowed to dry to provide an abrasive coating as a hard shell. This operation may be repeated to provide several layers. Drying is customarily done under controlled temperature and humidity conditions over several days for optimum results. When dried, the hard shell is cracked by repeated blows until it is conformable. While the resultant wheel has an acceptable cut rate and produces a desirable finish throughout its life, it has a number of disadvantages. A major disadvantage is the fact that the abrasive mineral is only present as a thin layer on the peripheral surface of the wheel, rather than existing throughout the wheel. Thus, when one area of the wheel's abrasive surface wears away, the entire abrasive coating must be replaced to produce an adequate abrasive product. Set up wheels are also very sensitive to use modifications by particular operators and may also be affected by changes in humidity, particularly if moisture-sensitive adhesives such as hide glue are employed.
While several attempts have been made to produce abrasive products to replace coated abrasive products and set up wheels for the aforementioned two applications or for other purposes, they have generally been not without disadvantage. The following is illustrative of the prior art in this regard.
U.S. Pat. No. 3,982,359 (Elbel) describes an abrasive wheel comprised of abrasive grain rigidly bonded together in aggregates which are then bonded in a resilient elastomeric matrix where the aggregates do not interfere with each other during movement under grinding conditions.
U.S. Pat. No. 2,216,728 (Benner et al.) describes bonding together aggregates composed of bonded abrasive particles to form a dense abrasive article.
U.S. Pat. No. 2,986,455 (Sandmeyer) discloses abrasive articles made with an abrasive component in the form of a hollow spherical or globular abrasive particle held together in a bonding matrix.
U.S. Pat. No. 3,048,482 (Hurst) discloses forming an abrasive article from a multiplicity of individually rigidly bonded abrasive bodies mounted or supported in a surrounding resilient matrix or reticulum in such a way that the rigid abrasive bodies can be described as being hinged to the ribs of the reticulum.
U.S. Pat. No. 3,871,139 (Rands) discloses a rotary abrasive hone made of multiple outwardly extending plastic bristles having enlarged abrasive globules firmly attached to the outer ends of the bristles.
U.S. Pat. No. 3,955,324 (Lindstrom) discloses a grinding tool comprised of abrasive agglomerates consisting of abrasive grains embedded in a metal phase and the agglomerates embedded in a synthetic resin.